Conventionally, as a brush material for use in a brushed motor, there is known a metal-graphite brush material for motor, manufactured by mixing graphite particles and copper particles together with using a binder solvent and then sintering the resultant mixture (see Patent Document 1).
According to one example of such method of manufacturing a metal-graphite brush material for motor, the method comprises: kneading particles of natural graphite with using a phenol resin solution as a binder; granulating the resultant material into a predetermined shape; mixing the resultant graphite particles with an amount of copper powder according to a density of current to be supplied to the brush, and with a required amount of solid lubricant; forming the resultant mixture into a predetermined shape; and sintering the resultant formed product under a non-oxidizing atmosphere, with blocking oxygen. With such manufacturing method, the phenol resin formed as a coating on the surfaces of the graphite particles is carbonized into non-crystalline carbon, thereby binding the graphite particles together. And, in the course of the reduction-sintering step, oxygen atoms and hydrogen atoms constituting the phenol resin dissolved solution are sublimed as carbon dioxide or steam, a number of pores will be formed on the surface and in the interior of the sintered product.
In general, with a motor using a metal-graphite brush, as the metal-graphite brush comes into sliding contact with the commutator, electricity is supplied from the brush to the commutator or from the commutator to the brush. And, to the commutator, there is connected a coil wound about a core provided in the rotor. In response to supply of electricity to the coil, the rotor is driven to rotate by a force of attraction or repulsion generated in cooperation with a permanent magnet disposed within a housing in opposition to the rotor.
The metal-graphite brushed motor, as being operated according to the above-described operational principle, suffers the problem of wear-out of the metal-graphite brush at its surface sliding against the commutator, due to the sliding contact between the metal-graphite brush and the commutator during driving of the motor. To restrict such wear-out of the metal-graphite brush during motor driving, various researches have been conducted so far.
As one example of such technique as above, there is a technique which takes note of the fact that the wear of the metal-graphite brush is attributable not only to the mechanical wear due to its sliding contact with the commutator, but also to electric load from spark discharge. Then, the technique proposes a metal-graphite brush material having a group of mutually contacting copper particles supported on the surfaces of the graphite particles (see, e.g. Patent Document 2). With this metal-graphite brush material, conductive paths for conducting electric charge induced from the graphite particles are formed on the surfaces of the graphite particles, so that spark discharge can be restricted, and the wear due to the spark charge too can be restricted. Further, with this metal-graphite brush material, as copper is formed into fine particles, it is possible to reduce the amount of charge discharged form the copper fine particles, thereby restricting the electric energy of the spark discharge and restricting also the level of electric noise generated during spark discharge.
According to a method of manufacturing such metal-graphite brush material described above, first, solution of copper complex is applied to the surfaces of the graphite particles to form a coating on the surfaces of the graphite particles. Then, these graphite particles are kneaded with a resin solution as a binder. The resultant graphite particle mixture is formed into a formed product. This formed product is then subjected to a sintering step under an oxygen-containing atmosphere and then a heat treatment under a reducing atmosphere. With this, copper particles are formed on the surfaces of the graphite particles and at the same time the graphite particles can be bonded together with the powder having a low molecular weight formed as the result of pyrolysis of the resin.
Patent Document 1: Japanese Patent Application “Kokai” No. 2001-298913
Patent Document 2: Japanese Patent Application “Kokai” No. 2005-12957